Introduction

Assessment of natural radioactivity in populated area surrounded by mountains is of great importance. Environmental radioactivity monitoring leads to evaluating levels and estimating radiation exposure for the inhabitants. It is also useful to gather long-term information on the levels, spatial distributions and trends. Uranium and thorium are useful in nuclear industry if exploration of them intended. However, their presents at high levels may have impact on human health due to internal and/or external exposures. Therefore, there are so many studies conducted at many places worldwide (example of that, the work done by: Paschoa and Godoy 2002; Sartandel et al. 2009; Al-Sulaiti et al. 2012; Saïdou et al. 2011; Popov 2012; Bezuidenhout 2013; Keith et al. 2015 and Idriss et al. 2016). Radioactivity measurements to produce baseline data such as maps and spread sheets are useful for a country in scientific manners. Natural radioactivity in the environment arises mainly from uranium and thorium series in addition to radioactive potassium (238U, 232Th and 40K). These nuclides pose risks from external exposure due to their gamma-ray emissions, and it can reach very high levels, especially if processed for some industry, e.g., producing fertilizers from phosphate rocks or mining (Makweba and Holm 1993; Sam and Holm 1995; do Rio et al. 2002), which may lead to high exposures to man. Some high-background radiation areas were identified around the world, and many of these high areas located at high altitudes (Mehdi 2013; Aliyu and Ramli 2015; Pooya et al. 2015; Sahoo et al. 2015). That motivated us to continue our radioactivity monitoring program and focus on high places such as Marrah mountain series that is located in Darfur region, western Sudan. Up to our knowledge, this is the first study of its kind intended to assess radioactivity profile in that region. The geology of the area is mostly characterized by various volcanic rocks (called Deriba Complex). The total area of Marrah mountain is about 75,000 km2 and reaches a height up to 3 km where high rainfall and water springs originate. The area is populated by approximately 1.5 million of inhabitants (majority are farmers and herdsmen) spend most of their time outdoors. Therefore, this study was carried out to characterize and evaluate the levels of natural radioactivity, assess external and effective dose to inhabitants due to gamma radiation in Marrah mountain and to construct map of ambient dose as baseline data for the country.

Materials and methods

Figure 1 shows the study area in the map of Sudan where the location of 103 sites recorded using a portable global positioning system (GPS). The area is located between (12.00°N–14.21°N) and (22.25°E–25.27°E) with altitudes ranging from 644 to 1999 m above sea level. Geological map of the area is shown in Fig. 2 (extracted from Geological Interpretation Map of Sudan 1:100 000), where main geological classes are identified. Ambient dose measurements were taken using two gamma survey meters Identifinder™ and RADOS. The Identifinder instrument functions were based on combined NaI detector (energy range 15 keV–3.0 MeV) and GM detector (energy range 60 keV–1.6 MeV). Beside its function of identifying nuclides, it gives measurements of dose rates (with total dose rate ranging from 0.01 µSv/h to 1 Sv/h). RADOS measures dose rates based on ionization chamber detector (ranging from 0.01 μSv/h to 100 mSv/h). The activity measurements of soil and rocks were taken using standard gamma spectrometry (located at Radiation Safety Institute of Sudan Atomic Energy Commission) where 238U, 232Th and 40K nuclides are evaluated. From location, rock samples were taken and soil samples were collected using a 10-cm-diameter auger, designed to sample up to 20 cm depth of soil. Two or three soil samples (wherever possible) were collected from each location, put into the plastic bags to ensure that cross-contamination does not occur, sealed, labeled and secured. The samples identification, location, date and time have been recorded on a logbook. Samples were then crumpled into homogeneous small pieces, sieved (through 2 mm sieve), placed in 500-ml Marinelli beakers and stored 3–4 weeks prior to measurement by gamma spectrometry. The specific activities were evaluated with gamma-ray spectrometry by the p type high-purity germanium (HPGe) using Gamma 2000 software and 3″ × 3″ sodium iodide (NaI) scintillation detector systems. Gamma spectrometry system employed for measurements in the present study was composed of 3″ × 3″ NaI(Tl) detector with built-in preamplifier, amplifier and MCA plugged to a computer through USB connection. The spectra are collected and analyzed using “Win TMCA32 target” GmbH software. The HPGe detector, with a relative efficiency of 20% (FWHM of 1.8 keV at 1773 keV), was calibrated using standard from Amersham™ and validated with the IAEA certified Reference Materials RGU-1 and RGTh-1. The activities of 226Ra, 232Th and 40K were computed from the 214Bi 609 keV, 228Ac 911 keV and 1460 keV gamma lines, respectively. The activity of 238U in each sample was estimated based on secular equilibrium assumption. The minimum detectable activity, for a 1000-min counting time, ranged between 0.5 and 10 Bq kg−1, for the radionuclides 214Bi, 228Ac and 40K.

Fig. 1
figure 1

Map of Sudan showing the locations of the samples of the present study

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Fig. 2
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Geological map of the surveyed area

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Gamma absorbed dose rates in air (at 1 m height from the surface), giving in nGy/h are calculated from the concentration of the three nuclides using the following equation (UNSCEAR 2000):

$$D\,({\text{nGy/h}}) = 0.461A_{\text{U}} + 0.623A_{\text{Th}} + 0.0414A_{\text{K}}$$
(1)

where A U, A Th and A K are activity concentrations (given in Bq/Kg) of 238U, 232Th and 40K, respectively.

External hazard index, H ex, from nuclide concentrations calculated using the formula is given in Eq. 2 as the hazard of the natural gamma radiation (Beretka and Mathew 1985):

$$H_{\text{ex}} = \frac{{A_{\text{U}} }}{370} + \frac{{A_{\text{Th}} }}{259} + \frac{{A_{\text{K}} }}{4810}$$
(2)

Based on the dose values obtained and locations of each sample, surface map of dose rates were created using geostatistical predicting tools in a geographic information system (GIS) software.

Results and discussion

The goals of the present work were to perform a first-time survey of natural radioactivity measurements of Marrah mountain series, which is located in Darfur region (western Sudan), relate radioactivity to geology and evaluate radiation dose received by inhabitants of the region due to presents of natural radionuclides. Geology of the area is mainly characterized by Deriba Complex—volcanic rocks. Deriba Caldera was formed by explosive eruption of the Marrah mountain volcano about 3500 years ago. The geological formations gave us the motivation to carry out this study where we expect to find spots of high background radiation. The results of 103 measurements taken by portable gamma survey meters and laboratory measurements for collected soil and rock samples (measured using gamma spectrometric techniques) of 238U, 232Th and 40K are presented in tables and figures. Table 1 shows the results of ambient dose measurements categorized according to seven identified geological classes. The highest value (0.25 μSv/h) was observed in Saur-Dalda-Fada complex, while the lowest value was in Alkaline Sand (0.05 μSv/h). Among the geological classes, Deriba complex showed the maximum average value of 0.20 ± 0.02 μSv/h, and sedimentary rocks showed the average value of 0.13 ± 0.02 μSv/h as the minimum average. Ambient dose rates data showed non-normal distribution when tested for normality as shown in the frequency distribution diagram (Fig. 3). Figure 4 shows the ambient dose results in relation to the geological classes given in Table 1. The absorbed dose rate in air values was in the range from 0.05 to 0.25 µSv/h with mean value of 0.18 ± 0.05 µSv/h. At altitudes greater than 1500 m, relatively high values were observed (0.18–0.22 µSv/h), while at low altitudes, the dose varies between 0.05 and 0.21 µSv/h as presented in Fig. 5. Relatively high values were observed in and around areas characterized by local flow and basaltic flow lithology. The low dose values, however, were observed at basement rocks (granitic) in general. The highest reading (0.25 µSv/h) was in Deriba Complex Pumice air-fall deposits (younger explosive phase).

Table 1 Ambient dose and calculated annual effective dose from different geological backgrounds
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Fig. 3
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Histogram of ambient dose rates

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Fig. 4
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Results of field measurement of ambient dose (μSv/h) versus main geological classes

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Fig. 5
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Dose rates at different altitudes

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Analyses of soil and rock samples showed the concentration of 238U, 232Th and 40K, ranging 12–80, 12–66 and 120–937 Bq/kg, and mean values, together with their respective standard deviation (SD), of 41 ± 6.9, 24 ± 12 and 540 ± 99 Bq/kg, respectively. These results are sorted and presented in Table 2 in association with geology where seven geological classes have been identified. The maximum values of uranium (80 Bq/kg) and thorium (66 Bq/kg) are comparable to maximum reported values from other regions (examples of that are reported by Selvasekarapandian et al. 2000; Jeevarenuka et al. 2014; Malczewski et al. 2004; El-Arabi et al. 2006; Abd El-mageed et al. 2011; Alnour et al. 2012; Beamish 2014 and Yildiz et al. 2014). In order to evaluate exposure, the activity concentration of these nuclides was then converted into absorbed dose rates using Eq. 1. Table 3 shows the average absorbed dose rates ranging from 23 to 142 nGy/h of 66 ± 20 nGy/h. The reported mean dose rates from the above-mentioned regions are also comparable to mean value found in this study. However, the maximum value of this study is higher than reported values from those regions [98 nGy/h in northern Tanzania (Banzi et al. 2002), 55 nGy/h in Egypt (El-Arabi et al. 2006), 112 nGy/h in Malesia (Alnour et al. 2012), 74 nGy/h in India (Jeevarenuka et al. 2014) and 135 nGy/h in Kashmir (Rafique et al. 2014)]. The maximum value of absorbed dose rate found in this study (142 nGy/h) is lower than the maximum reported value (300 nGy/h) for granitic terrain areas in UK (Beamish 2014). The mean and maximum values of the dose rates were higher when compared to the world average (59 nG/h) and maximum (93 nGy/h) given in the United Nations Scientific Committee on the Effect of Atomic Radiation report (UNSCEAR 2000).

Table 2 Descriptive statistics of the concentration of 238U, 232Th and 40K (Bq/kg) in rock and soil samples, calculated absorbed dose rate, D (nGy/h) and external hazard risk, H ex, for all geological classes (summary)
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Table 3 Average absorbed dose rate, D (nGy/h), calculated from radionuclide concentrations and average external hazard risk, H ex, for each geological class
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The wide range of absorbed dose was found in this study to express the complexity of the radioactivity distribution in the area. In order to see trends, an attempt was made to construct a surface map of gamma radiation dose by creating surface where estimation of unmeasured places was predicted using a built-in geostatistical tool of kriging in GIS software. Cross-validation performed for this estimation exhibited a root-mean-square error of 23 nGy/h and average standard error of 24.5 nGy/h. Salih et al. 2002 reported the details of the prediction methods (kriging, inverse distance weight, etc.) needed to create a continuous surface from scattered data points. Figure 6 shows the results of the prediction where high levels were observed in areas characterized by basaltic lithology (as compared with Fig. 2).

Fig. 6
figure 6

Surface map showing the spatial distribution of ambient dose predicted from the measurements giving values range from 0.05 μSv/h (light color) to 0.25 μSv/h (dark color)

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To limit the external γ-radiation dose and asses hazard risk, the model proposed by Beretka and Mathew (1985), as given in Eq. 2, was used in this study. Table 2 shows the values range between 0.1 and 0.67 with a mean value of 0.32 ± 0.09, which is less than unity indicating low risks of exposure due to the presence of studied radionuclides at these levels. Table 3 shows average external hazard risk (H ex) categorized according to the geological classes.

The inhabitants of this region spend most of their time outdoors; therefore, the dose calculated here presents actual exposures. (Indoor do not possess additional exposure because most of the houses are built of local forests materials.) Annual effective dose rate in air is estimated using the conversion coefficient of 0.7 Sv/Gy (UNSCEAR 2000), and the effective dose equivalent (D E) is then given by Eq. (3):

$$D_{\text{E}} \,(\upmu{\text{Sv/y}}) = D\,({\text{nGy/h}}) \times 8766 \times 0.7 \times 10^{ - 3}$$
(3)

The calculations reveal the values of D E ranging from 0.14 to 0.87 (0.40 ± 0.12) mSv/y. However, the average annual effective dose calculated from ambient dose measurements (Table 1) for the population is 1.4 ± 0.4 mSv indicating relatively high values. The average values from the study area are higher than the worldwide average for outdoor annual affective dose, 0.07 mSv per year, as reported by UNSCEAR (2000).

The overall results suggest that radiation levels in the area are slightly high and that the created map will help to identify areas of high levels in any future studies.

Conclusion

Ambient dose (dose rates at 1-m above the ground) and natural radioactivity of soil–rocks were investigated in Marrah mountain, western Sudan. The results obtained indicated that the levels of 238U, 232Th and 40K ranges (mean ± SD) were 12–80 (41 ± 6.9), 12–66 (24 ± 12) and 120–937 (540 ± 99) Bq/kg, respectively, resulting in equivalent total gamma absorbed dose rate of 23–142 (66 ± 20) nGy/h. The ambient dose ranged from 0.05 to 0.25 µSv/h with a mean value of 0.18 ± 0.05 µSv/h. The average annual effective dose for the inhabitants was calculated to be 1.4 ± 0.4 mSv. The data presented in this survey give a first preliminary result obtained from Darfur region, which is useful as a guide for further studies. Geographic information system (GIS) to create surfaces is found useful to identify trends.